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Do the World?s Oceans Take Up Less Carbon Dioxide?
19 Nov, 2007 11:33 am
The answer is: Yes, for the North Atlantic over the last decade.
Our data come from merchant ships carrying bananas from the West Indies to the UK. The instrument onboard MV Santa Maria, chartered by Geest Line, UK, has generated more than 90,000 measurements of CO2 in the past few years alone. We can show the dramatic decrease in the uptake by the North Atlantic by the comparison with measurements made onboard a similar merchant ship on the same route in the mid 1990s. Measurements made further north, by French and American colleagues onboard merchant ships trading between Iceland and the USA, indicates a similar decrease in the uptake of atmospheric CO2 over the last decade (Corbière et al., 2007). Other measurements in the Southern Ocean show that here also less CO2 was taken up from the atmosphere in 2004 than in the early 1980s (Le Quere et al., 2007).
Before the industrial revolution, atmospheric CO2 levels were about 280 parts per million (ppm) – in 2006 they were about 380 ppm. This is now at a level higher than it has been for more than 650,000 years (Petit and al, 1999; Siegenthaler et al., 2005). The increase in atmospheric CO2 levels in itself is caused mainly by the burning of fossil fuels. These fossil fuels, including coal, gas, and oil, contain massive amount of carbon (the “C” in CO2), which for millions of years has been buried deep within the Earth. They had thereby been removed from the biological cycle on the Earth surface. We are now digging up this fuel and burning it, releasing the carbon as CO2, at a vastly faster rate than it would have happened naturally. And these additional human CO2 emission increase CO2 in the atmosphere.
However, up to now about half of all human CO2 emissions have been taken up by the world’s oceans. In this way, the oceans act as a strong ‘sink’ for human emissions, without which CO2 levels would have risen much faster in the atmosphere than they actually have.
A recent global study by colleagues at UEA together with an international team (Canadell et al., 2007), shows that the fraction of CO2 emission taken up by the oceans has decreased since around 1960. Our result from the North Atlantic verifies this by direct measurements. Canadell and co-workers used global human emission data and mathematical modelling techniques, showing that CO2 in the atmosphere does not only increase, but this increase has accelerated since the year 2000. This is partly due to more fossil fuel being burned, and partly due to the increased use of coal over oil and gas, releasing more carbon dioxide per unit of energy produced.
The question is how much the North Atlantic, and the global oceans for that matter, will take up in the future. Computer models of ocean uptake vary in their conclusions: from more uptake, to no change in the uptake, to less uptake.
One reason is that the underlying processes controlling the uptake of atmospheric CO2 by the oceans are complex, and are extremely difficult to predict. They include physical, chemical, and biological conditions in the seawater itself, as well as atmospheric condition, and they are interlinked. For example, much of the climate over the North Atlantic is governed by a phenomenon called the North Atlantic Oscillation (NAO). It describes the changing pressures systems across the North Atlantic over years and decades. Atmospheric pressure systems create winds, these winds in turn influence the surface ocean currents, which in turn influence the chemistry, physics, and biology of the ocean surface, which in turn influence the uptake of CO2 from the atmosphere, as well as the atmosphere itself again. The NAO increased from the 1960s to the early 1990s, then fell again until 2005, and was high again in 2006/2007 (Osborn, 2007). The future of the physical, chemical, and biological properties within the oceans, as well as phenomena such as the NAO, will all influence the uptake of atmospheric CO2 in the future.
Our research does show that the North Atlantic has definitely taken up less CO2 from the atmosphere in the early 2000s than in the mid-1990s. We hope to be able to continue our measurements, to determine the future uptake of CO2, and how this sink is controlled, since if the trend of the last decade continues, the ocean will take up less and less. However, if the underlying processes controlling the uptake change in the future, the ocean may take up more again.
1. Canadell, J.G. et al., 2007. Contributions to accelerating atmospheric CO2 growth from economic activity, carbon intensity, and efficiency of natural sinks. Proceedings of the National Academy of Science, USA, 104(24): 10288-10293.
2. Corbière, A., Metzl, N., Reverdin, G., Brunet, C. and Takahashi, T., 2007. Interannual and decadal variability of the oceanic carbon sink in the North Atlantic subpolar gyre. Tellus, 59B(2): 168-178.
3. Le Quere, C. et al., 2007. Saturation of the Southern Ocean CO2 sink due to recent climate change. Science, 316(5832): 1735-1738.
4. Osborn, T., 2007. North Atlantic Oscillation index data. http://www.cru.uea.ac.uk/~timo/projpages/nao_update.htm.
5. Petit, J.R. and al, e., 1999. Climate and atmospheric history of the past 420,000 years from the Vostok ice Core, Antarctica. Nature, 399: 429-436.
6. Schuster, U. and Watson, A.J.W., 2007. A variable and decreasing sink for atmospheric CO2 in the North Atlantic. Journal of Geophysical Research, 112: C11006, doi:10.1029/2006JC003941.
7. Siegenthaler, U. et al., 2005. Stable carbon cycle-climate relationship during the late Pleistocene. Science, 310(5752): 1313-1317.